<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    p {
      /*
        text-decoration 文本装饰
         ● none
         ● overline      上划线
         ● line-through  删除线
         ● underline     下划线
       */
      text-decoration: underline;
    }
  </style>
</head>

<body>
  <p>Hello world!</p>
</body>

</html>